The invention relates to an arrangement for capturing an image of a printing substrate web in a printing machine, comprising a camera that has a sensor field extending parallel to the printing substrate web, and an objective lens an optical axis of which runs at right angles to said sensor field, and comprising a light source that is arranged relative to the printing substrate web such that the light emitted therefrom is reflected by reflective surface portions of the printing substrate web mainly into the camera.
An image capturing arrangement of this type serves for example for generating a feedback signal for a register controller in a printing press. The light source shall be arranged so as to achieve a good image contrast, in particular also in those cases where the printing substrate web has a reflective surface or a least reflective surface parts, for example, when the printing substrate web is a reflective foil or is metallised or coated with a reflective lacquer.
For obtaining a high contrast between background (printing substrate) and the printed image, there exist on the one hand bright field illuminations wherein all reflective parts appear darker in the image, and on the other hand so-called coaxial illuminations wherein the beam paths from the light source to the printing substrate web and from the printing substrate web to the camera extend coaxially so that the light from the light source is reflected directly into the camera and consequently the reflective parts appear bright. Such a coaxial illumination has heretofore been achieved by means of semi-transparent mirrors, because otherwise the camera would obscure the light source or, conversely, the light source would obscure the camera. However, arrangements employing semi-transparent mirrors suffer from high losses in light intensity.
Another possibility is to arrange the light sources on a circle around the optical axis of the objective lens of the camera. In that case, however, a homogeneous illumination of the image field cannot be obtained.